In recent years, the data capacity of images and motion pictures processed by portable terminals are increasing drastically. For this reason, data transmission speed between data holding devices such as a portable device and a PC becomes bottlenecks.
A technique for solving such problem of data transmission speed between devices includes an optical transmission technique for replacing an electric signal with an optical signal. Such optical transmission technique includes a type using an optical transmission path such as an optical fiber and a type for transmitting light in the space.
The optical fiber transmission is generally used for long distance transmission, but in order to connect devices, it is necessary to improve the precision of constituent components so as to improve alignment precision of connection units. Therefore, there is a defect of high cost and fragileness.
On the other hand, the space transmission does not require none of such constituent components, and can simplify the transmission/reception unit. However, it is necessary to receive transmission light by reducing the loss as much as possible. With regard to this point, it is an object to improve the alignment precision like the optical fiber transmission.
One of the solutions includes making one or both of the transmission/reception into an array. This would improve the tolerance of alignment deviation, but there is a problem in that the size of the constituent component increases. There is a technique called beam steering for changing the direction of the optical output, but there is a problem in that the size of the devices that have been suggested until today are extremely large.
On the other hand, there is a technique that uses semiconductor manufacturing process to form an optical transmission path. In this technique, by using silicon (Si) photonic wire waveguide in which the contrast of the refractive index between the core and the surrounding portion is high, the size of the optical device can be reduced. Typical cross sectional size of the Si photonic wire waveguide of wavelength 1.55 μm band is 220 nm×450 nm. Due to strong optical confinement effect based on high refractive index difference, even a bent waveguide of which curvature radius is small can achieve a low level of radiation loss. When highly developed CMOS process technique is applied, optical integrated circuits obtained by integrating many microscopic optical/electronic devices can be mass produced. Therefore, this is expected to be applied to not only optical inter connection between equipments and between boards but also high capacity optical wiring between chips and in a chip.
In particular, in an intrachip optical wiring in which wiring length is several centimeters or less, a simple optical parallel wiring, that is, Space Division Multiplexing (SDM), is more advantageous in terms of smaller overhead, cost, power consumption, and the like, rather than using Wavelength Division Multiplexing (WDM) requiring strict wavelength management or Time Division Multiplexing (TDM) requiring optical transmission/reception circuits and SerDes of several dozen Gbps. In the SDM, the number of wirings increases, but if necessary, by increasing the wiring layers, the density of wirings per area can be increased. In any case, when multiple points in a chip are connected with optical wiring, at least two layers of optical wiring layers are required to cross the waveguides.
It is not practical to achieve multilayer optical wirings by pasting expensive silicon-on-insulator (SOI) substrates, and therefore, multiple optical wiring layers are preferably fabricated using backend process of CMOS just like electric wiring layers. A photonic wire waveguide formed using hydrogen-terminated amorphous silicon (a-Si:H) deposited by plasma CVD at a low temperature of about 300 degrees Celsius on SiO2 has already realized low loss optical propagation property that is as good as a photonic wire waveguide formed with the top crystal Si layer of a SOI substrate.
Examples of optical couplers optically connecting optical wiring layers of multilayer optical wirings include a pair of inversely taper waveguides, a directional coupler, and a multi-mode interference (MMI) optical coupler.
The optical coupler using the taper waveguide include an incidence side waveguide configured to be narrower in a tapered manner and an output side waveguide configured to be wider in a tapered manner, which are arranged in proximity to each other so as to overlap in the vertical direction. The coupling efficiency can be maintained at a high level even if there is some deviation in the position, but in order to change the wave guiding mode in a continuous manner, a long taper is required, which makes it difficult to reduce the size.
The coupling length of the vertical directional coupler can be somewhat reduced when the wiring layer spacing is narrowed, but in this case, tolerance is also reduced, and therefore, even a small deviation in the position or asymmetry (difference in the width, thickness, refractive index, and the like) between the two optical waveguides deteriorates the coupling characteristics seriously.
The tolerance of the MMI optical coupler is relatively high even if the size thereof is reduced, and therefore, it is generally used as an optical coupler within a plane using the Si photonic wire waveguide. A vertical MMI optical coupler used for connecting wiring layers has been suggested, but a specific method for making a three-dimensional structure including upper/lower input/output waveguides has not yet been disclosed. In particular, in an on-chip optical wiring, it is desired to develop a technique for fabricating a small vertical MMI with a width of sub-micron and with a length of about 10 μm with a high degree of precision, using a-Si material. Even if a high precision stepper is used, the alignment accuracy is about ±20 nm, and therefore, it is desired to make a structure that can suppress excessive loss as much as possible even with this level of offset.